Julian Edgar, 50, has been writing about car modification and automotive technology for nearly 25 years. He has owned cars with two, three, four, five, six and eight cylinders; single turbo, twin turbo, supercharged, diesel and hybrid electric drivelines. He lists his transport interests as turbocharging, aerodynamics, suspension design and human-powered vehicles.
Cost vs benefit of car modifications
When modifying cars, everyone conducts some sort of cost/benefit analysis.
That might be as informal as weighing-up the likely cost of the modification against the guessed benefit, or it might be a more detailed analysis.
A friend of mine, Paul, has a rule of thumb that goes like this:
Back in 1998, on naturally aspirated cars, he budgeted $100 per kilowatt for a power improvement. Any more than that and he thought the value poor; any better than that and – well, he thought that was pretty good.
That $/kW ratio was for mods like intake, exhaust and chip.
So a new commercially-produced intake to an airbox might cost $600 – he wanted at least 6kW from it. An exhaust might cost $1000 – so at minimum, a 10kW gain. And ditto with the chip.
In fact, for many people, that cost/benefit ratio is still pretty much on the money.
Time passed, and by 2002 Paul was into Porsches. On his Porsche 993, that ratio jumped to $200/kW for modifications like exhaust, turbos, engine management re-mapping and an oil cooler. (Well, the last couldn’t be quantified in $/kW, but you get the idea.)
By 2007 the car had changed to a Porsche 996 turbo and the ratio for an exhaust and management re-map had dropped to $180/kW – and for a remap alone, just $66/kW.
Another, different, way I’ve seen of defining a cost/benefit analysis is to suggest that if a power improvement is less than 10 per cent, the driver won’t be happy.
Why? Because they won’t be able to feel the change in performance.
This idea was suggested to me by a manufacturer of extractors and exhausts: if in dyno testing, they couldn’t get a 10 per cent power improvement, they didn’t sell the product. Why? Because they knew that irrespective of the cost to the customer, the customer would be back with complaints.
Of course, quantifying performance improvements in terms of peak power figures alone is flawed – what about part throttle response, bottom-end torque (and so driveability), fuel economy and so on?
Take into account these factors and the cost/benefit becomes very personal. If you are doing the modifications yourself, the ‘cost’ part of the modification may also heavily involve individual effort.
I have been thinking of these ideas after publication of three stories in AutoSpeed, all of which are based on modifications I have made to my Honda Insight.
In order of publication, they are The 5 cent Modification, Tweaking the EGR Part 1 and Part 2, and Trialling a Rear Undertray.
The 5 Cent Modification used a resistor to alter the output signal of the intake air temp sensor. It was a modification that was incredibly cheap and very simple to fit, with minimal testing needing to be undertaken.
Any benefit at all from this modification and I’d have been happy – but with the much improved driveability, the modification is probably worth $500 to me. $500 versus 5 cents, and with only a few hours needing to be spent, is a pretty good cost/benefit ratio!
But Tweaking the EGR was a much more borderline modification. This modification changes the amount of exhaust gas recirculation that occurs at part-throttle. Again it was cheap and pretty easy modification to install, but setting it up and then proving its worth took a lot of testing.
I think it’s a good modification – especially as it improved the fuel economy on a car with already exceptional fuel economy – but personally, it’s worth to me perhaps only $200. But again, that’s still a pretty positive result.
The undertray, though, was a different story. The first step was to tape into place a plastic trial undertray and then do some testing. Installing the trial undertray took about three hours; testing it took literally hundreds of kilometres of continuous freeway driving.
When I previously did an undertray on a Toyota Prius, the improvement was noticeable pretty well straightaway. With the Honda, I couldn’t detect any change at all, either in fuel economy or car feel.
As has been suggested to me, I could have then trialled a differently shaped undertray; I could have changed the way the leading edge was organised; I could have used vortex generators on it; I could have made different wheel fairings.
But instead I chose to take it off!
The undertray was a good example of where for me, the cost – especially in time being spent – heavily outweighed the benefit. To be worthwhile continuing, the trial undertray should have made a clear change to the way the car performed – whether that was to make things worse or better.
No change at all and I couldn’t see any point in persisting…
on November 27th, 2008 at 10:54 am
My ecu reflash for my Polo GTi ended up being approx $45/kW… interestingly it works out at about $14/Nm.
on November 28th, 2008 at 6:12 pm
I have recently reached this point with my project. After changing engines, suspension and brakes I was very pleased with the improvement, however I’m now wondering where to go with it. It’s easy enough to get more power etc, but the whole cost/benefit thing is really coming into play now. It’s an older car so that weighs even more heavily. Consequently I’m now aiming for mods that deliver a clear benefit for the $ spent. (hence my presence on the Autospeed website!) Only so far one go with all that though. One thing’s for sure, whether you buy new(er) or fix up an oldie they still cost you lots of $.
on December 8th, 2008 at 9:24 am
Speaking of Vortex generators – the new Ford Focus might do well with the rear sunshade / spoiler removed and replaced with a couple of vortex generators?
on December 8th, 2008 at 9:28 am
Hmm something which would save everyone money experimenting would be a list of vehicles and known performance mods.. anyone know of such a thing? I guess the problem would be keeping it objective..
on January 1st, 2009 at 4:45 pm
It is getting harder & harder to modify standard production vehicles. Even high performance production cars like the STI, EVO X and the new GTR have now got fail-safe electronic gadgets that prevent re-mapping/re-flashing of the ECU. Even replacing air-filter pods and exhaust systems no longer have the power effect that it used to have. In short, there is no more further horsepower gains to be expected when you buy a STI, EVO X or GTR.
Even the simplest and cheapest modification – changing the air-filter box to the air-filter pod – no longer have the effect that it used to have! Simply, engine management system have become so sophisticated that intake air is measured precisely at the source. If fact, the air-filter pod may caused engine damage as the air flow may caused turbulence at the metering source or the increased air-flow may disrupt the air-fuel program – thus the ECU may either have to enrich the air-fuel mixture to compensate [causing poor fuel economy or engine wear] or lean out the air-fuel mixture due to increased air-flow [causing engine overheating or piston damage]. Today, air-filtration modification may even caused a check engine light [CEL] to flare up or even caused the car to operate on a limp home mode. Even exhaust mods in a modern car can caused a CEL or limp-home mode – this can lead to your car’s warranty being revoked.
The thought of people still modifying their engine cylinder heads via head porting, skimming or even big-valve modifications are simply ridiculous in this age. The engine have come very far with CAD/CAM production methods, better metallurgy and even computer engine simulation. Cylinder head mod were popular during the British era of manufactured cars – remember the Mini Cooper, Hillman Imp, Lotus Cortina, etc. A particular Englishman by the name of Richard Longman made his reputation via the Mini Cooper S – he sold a lot of modified ported, gas-flowed and big-valved Cooper S cylinder heads. But that was in the 1960s and early 70s! The modern day engine no longer respond to these mods – if fact, you would be sending the engine to the wreckers earlier!
Even early day turbo modification like tweaking the boost via mechanical boost controllers, replacing the cat downpipe with a cat-less straight thru downpipe [illegal BTW] or even modding the intake side of the turbo via changing of bigger turbo fins are simply passe. Today, these mods can effectively wreck your engine!
Simply, modern day modifications are big-bucks if you are thinking of competing in the Targa Tasmania or Targa West type of events. Otherwise, don’t waste your money in modding your car if you’re only driving on public roads.
Save your money to buy the best performance car you can afford if you like to have high performance and keep the car bog standard! Also, drive safely and sanely if you want to keep your license.
on January 1st, 2009 at 5:45 pm
Joe Public, nearly everything you have written in the above post is either outright wrong, mistaken or misleading.
Yes, it is harder to make changes that are effective, and those changes need much better tools and techniques – but that’s as far as it goes.
Perhaps you should read more widely (or read better material) before making these sorts of statements?
on January 4th, 2009 at 5:56 pm
To Julian Edgar:
In the real world, truth is never ‘..outright wrong, mistaken or misleading’ as you are claiming. It is somewhere out there but no one should be arrogant enough to claim the truth when there are so many variable and real-life situations.
Frankly, 80-90% of the general population are not so mechanically & electronically inclined nor are able to lift a wrench to fix our cars. We rely on our mechanics to tell us the problems and our tuners to advise us on our car modifications. As such, we learn from our workshop experiences and from our wallets. When we refer to Autospeed articles to our tuners, we are laughed at by them who proceed to tells us that your written articles are textbook crap, not real world stuff! The textbook stuff differs from the workshop experience – I was bluntly told! “…Perhaps you should read more widely (or read better material) before making these sorts of statements?” – an offensive statement considering that you do not know my educational status but I’ll let it go. In the workshop, whatever knowledge I have about cars is irrelevant except the mechanics’ knowledge. Everyone know that you can put in your 2 cents worth but the mechanics can overrule you in everyway!
Also, some of your modification stuff in Autospeed as I was told by these mechanics are not worth doing as the costs for doing it far exceed the benefits. As many of us are born with 2 left thumbs, DIY’ing is out of the question – thus leading to my question how many of the modification articles advocated by Autospeed are actually ‘do-able’?
on January 4th, 2009 at 6:30 pm
Joe Public – you haven’t defended any of your statements from the previous post – and indeed, how can you when most of them are basically indefensible? They are certainly not the ‘truth’, as even a cursory examination of current modification technology will show you.
I certainly don’t think it is appropriate to let such erroneous information go posted unchallenged – people might believe it!
How many of our AutoSpeed modification articles are do-able?
All of them.
Cite the ones that aren’t, or the ones where the costs exceed the benefits.
As for reading more widely, I certainly didn’t intend it to be offensive. From your statements you clearly haven’t been keeping up with car modification, and to remedy that situation, it seems an appropriate statement to say that you need more information. The best way of getting that is to read more widely.
on January 4th, 2009 at 8:19 pm
[OK, have more time now.]
It is getting harder & harder to modify standard production vehicles.
[Maybe. In actuality, in many cases it is far easier than it ever has been. In some cases it is harder]
Even high performance production cars like the STI, EVO X and the new GTR have now got fail-safe electronic gadgets that prevent re-mapping/re-flashing of the ECU.
[Wrong – the Sti and Evo X are actually two cars that can EASILY be re-mapped.]
Even replacing air-filter pods and exhaust systems no longer have the power effect that it used to have.
[We’ve never recommended replacing air filer pods (modifying the intake to the airbox works much better) and exhausts still give a power improvement on pretty well all cars, esp in conjunction with engine management changes.]
In short, there is no more further horsepower gains to be expected when you buy a STI, EVO X or GTR.
[Quite wrong]
Even the simplest and cheapest modification – changing the air-filter box to the air-filter pod – no longer have the effect that it used to have!
[In some cars even in 1984 you got problems if you did this…]
Simply, engine management system have become so sophisticated that intake air is measured precisely at the source.
[Huh? Intake air measurement has changed little since the mid Eighties.]
If fact, the air-filter pod may caused engine damage as the air flow may caused turbulence at the metering source or the increased air-flow may disrupt the air-fuel program – thus the ECU may either have to enrich the air-fuel mixture to compensate [causing poor fuel economy or engine wear] or lean out the air-fuel mixture due to increased air-flow [causing engine overheating or piston damage].
[In fact, the chances of this happening is less than ever these days with factory wideband oxy sensor feedback. But as I said above, disrupting airflow meter readings has been a problem for over 20 years.]
Today, air-filtration modification may even caused a check engine light [CEL] to flare up or even caused the car to operate on a limp home mode. Even exhaust mods in a modern car can caused a CEL or limp-home mode – this can lead to your car’s warranty being revoked.
[Well, I guess if the people doing the exhaust don’t have any idea. Mods have always revoked warranties.]
The thought of people still modifying their engine cylinder heads via head porting, skimming or even big-valve modifications are simply ridiculous in this age.
[No it isn’t. It’s just harder to get improvements and better tools are needed.]
The engine have come very far with CAD/CAM production methods, better metallurgy and even computer engine simulation.
[So? Does that mean no gains can be made on an individual basis? Not at all.]
Cylinder head mod were popular during the British era of manufactured cars – remember the Mini Cooper, Hillman Imp, Lotus Cortina, etc.
[All before my time of interest in cars – I only became interested when EFI was introduced.]
A particular Englishman by the name of Richard Longman made his reputation via the Mini Cooper S – he sold a lot of modified ported, gas-flowed and big-valved Cooper S cylinder heads. But that was in the 1960s and early 70s! The modern day engine no longer respond to these mods – if fact, you would be sending the engine to the wreckers earlier!
[Current engines respond to better breathing – it’s just harder to do. I certainly wouldn’t recommend head work as a first step, but to say it is invalid is simply not true.]
Even early day turbo modification like tweaking the boost via mechanical boost controllers,
[not true – it depnds on how it is done…]
replacing the cat downpipe with a cat-less straight thru downpipe [illegal BTW]
[when have we ever said to do this?]
or even modding the intake side of the turbo via changing of bigger turbo fins are simply passe.
[“fitting bigger turbo fins” has never been a achievable mod – they’d rub on the housings… Proper turbo changes are just as worthy as they ever have been.]
Today, these mods can effectively wreck your engine!
[Mods have always been able to wreck your engine.]
Simply, modern day modifications are big-bucks if you are thinking of competing in the Targa Tasmania or Targa West type of events.
[Some modifications can be done for literally cents.]
Otherwise, don’t waste your money in modding your car if you’re only driving on public roads.
[Why? People here like modifying cars!]
Save your money to buy the best performance car you can afford if you like to have high performance and keep the car bog standard!
[See above. If you don’t like modifying cars, why on earth are you reading AutoSpeed?]
Also, drive safely and sanely if you want to keep your license.
[Ah, finally a point I can agree with!]
on January 5th, 2009 at 5:44 am
^^^ What he said.
Another point that probably needs bringing up is that with all of the variable valve timing, electronic throttle, stability control etc. the ‘character’ of the car lies in a computer program. Either altering the program (at moderate cost) or the inputs to the computer (mere cents) can dramatically alter the car.
Take modifying the G- sensor input on the R32 GTR for example, the car goes from blazing cool looking (but slow) power slides into something that does what it should, and has far more traction.
http://www.autospeed.com/cms/A_107753/article.html
Complex computer control is only a downer until you get your head around how the computer makes it’s decisions.
Incedentally Joe, what’s with pod filters being mentioned so many times? Especially given that pod filters have had major downsides on most cars they have ever been put on. The only car I have ever considered putting a pod on (my EF falcon, with a MAP sensor) I elected not to because of the difficulty of getting only cold air to it.
on April 16th, 2009 at 8:44 pm
HI i just came across your site an i love your site.The first thing i read was about aerodynamics/front spoilers.Then i read about dynos and air /fuel devices.Regards aero stuff you asked if anyone has a design to produce downforce,I’ve put in 3 years on my aero design at the front and rear of my car to produce downforce at front and rear and to remove vortex and drag at the rear of car.Can you help me?My ideas are made and installed at present and i think they,re good enough to offer to a car manufacterer.You no us kiwi,s we,ve done some amazing breakthroughs.Contact me privatly by my email if your interested..I cauld type on and on about my breakthroughs in aerodynamics..but maybe in time i will,I also have many real ways around factory computors when motors are modified,With thanks Glenn.